1. Field of the Invention
This invention relates to pneumatic radial tires and a method of producing the same, and more particularly to an improvement of an uneven appearance in a sidewall face of the product tire.
2. Related Art Statement
Recently, tires are strongly directed to achieve high grade and high performance. To these ends, the demand for improving the appearance is ever increasing in addition to the improvement of running performances and durability performance.
In pneumatic radial tires for passenger cars and small size trucks, organic fibers such as rayon, nylon, polyester and the like are mainly used as a cord for the carcass ply. The use of such fibers in the carcass ply is generally carried out by the following method.
That is, the above fibers are twisted every 2 to 3 fibers while a constant twisting is applied to each of these fibers, whereby plural cords are obtained as a warp. Then, these cords are aligned and a fine and weak weft yarn is roughly driven in a direction perpendicular to the warp to form a tire cord fabric, which is subjected to an adhesive treatment for adhering to rubber. Thereafter, such a cord fabric is covered with a topping rubber at a constant thickness to form a rubberized cord fabric.
Next, the rubberized cord fabric is cut so that the warp has a constant length and the opposed edge portions of the cut cord fabrics other than the cut face are joined to each other to form a carcass material for radial tires.
In tire building, the above carcass material is cut on a tire-building drum (or similar equipment) in the same direction as the warp and joined at the cut ends into a cylindrical shape. Then, bead cores, belt, sidewall rubbers, tread rubber and the like are attached thereto, which are vulcanized to obtain a tire product.
In the joining step among the above steps, the opposed edge portions of the cord fabrics cut at a constant length in a direction perpendicular to the warp are overlapped to each other in the same direction as the warp on the building drum, so that the number of cords in the overlap portion is two times than that in the other portion (see FIG. 4a).
When internal pressure is applied to the tire product, the tensile modulus of the overlap portion becomes twice as great, so that the cords in the overlap portion hardly elongate. As a result, such an overlap portion forms a depression as an appearance of the tire product when the tire is inflated under a given internal pressure. When the depression amount is not more than about 1 mm, the depression is quiet, while when it exceeds 1 mm, the problem is raised as a commercial value. Recently, a case has been noted where the depression amount exceeding 0.5 mm is claimed as a tire appearance issue in the market. Therefore, various countermeasures for solving this problem have been attempted in the art. For example, there is proposed a technique wherein the cord used in the overlap portion has a modulus of elasticity lower than that in the other portion. Another technique proposed has the cords in one of the edge portions to be overlapped are cut at one or more positions to tensionly act the overlap portion as a single ply other techniques propose raising the modulus of elasticity of the cords as a whole, and having the edge portions joined to each other without overlapping, and the like.
However, all of the conventionally proposed techniques do not sufficiently solve the above problems.
In the technique of reducing the modulus of elasticity in the cord of the overlap portion, the improving effect of the overlap portion is developed at the cutting step of the rubberized cord fabric, but the opposed edge portions of the cut cord fabrics are not necessarily overlapped with each other at the tire building step, so that the improving effect as a whole of the tire product is insufficient. In this case, the low modulus cords are required to be arranged in only the overlap portion, so that it is difficult to control such an arrangement. If the low modulus cords are arranged in a portion separated away from the overlap portion, these cords rather elongate in a convex direction in the inflation of internal pressure, which raises a problem with respect to the durability in the tire comprising solely a carcass type material.
The technique of cutting the cords in one of the edge portions to be overlapped has similar problems. Furthermore, if it is intended to conduct this technique with a high accuracy, the reduction of productivity becomes conspicuous, so that there is still a problem in practical use.
In the technique of raising the modulus of elasticity in the cord as a whole, the depression tends to decrease in the overlap portion. As the method of raising the modulus of elasticity of the synthetic fiber cord, there is generally adopted a process of increasing the tension applied to the cord during the heat treatment. However, when such a process is conducted, the heat shrinkage of the cord too rises, which badly affects the shape stability, uniformity and the like of the tire, so that it can not be said that this method is a sufficient means.
Among the conventional techniques, the butt joining of cords without overlapping is a best method, but it is not put into practical use at the present because fine cords made from the organic fiber are used as a carcass material in tires for passenger cars or small size light trucks.